This proposal requests renewal for years 21-25 of the P01 program "Hormonal Signals that Regulate Ovarian Differentiation" at Northwestern University. The program has a long history of collaborative discovery-based research in the reproductive sciences that has led to numerous scientific breakthroughs over the lifetime of the grant. The major theme of the program is to investigate signaling pathways by which hormones or other regulatory factors act on the ovary to promote steroidogenesis and the maturation of follicles and germ cells necessary to sustain female fertility. To investigate this theme, 3 new research projects are proposed that represent novel and creative research directions inspired by our progress during the last grant period and by our most recent preliminary findings. Project 1 (Kelly Mayo and Teresa Woodruff) focuses on ovarian development and the events that regulate the breakdown of germ cell nests to form the initial pool of primordial follicles. These studies use novel approaches to investigate roles for Notch signaling, activin action and estrogen regulation leading to ovarian follicle formation. Project 2 (Jon Levine, Larry Jameson, Jeff Weiss) examines later stages of follicle growth, with an emphasis on estrogen signaling through non-classical ERa pathways both within the ovary and between the ovary and the hypothalamic-pituitary axis. These studies use creative approaches to dissect mechanisms of ERa signaling in both the brain and the ovary critical to reproductive function. Project 3 (Teresa Woodruff, Tom O'Halloran) explores the final stages of follicle and oocyte maturation and investigates the inorganic signaling molecule and transition metal zinc and its actions within the female germ cell. These experiments apply exciting approaches at the chemical-biology Interface to better understand the regulation of oocyte maturation. An Administrative Core and an Ovarian Histology Core provide support that facilitates the research in the 3 scientific projects. The collective strengths of this highly interactive investigative team allow for a synergism of ideas and research collaborations that will ensure that we progress well beyond the limitations of any individual project and capture the full advantages of the P01 program mechanism. Establishment of the initial follicle pool, control of follicle function and steroidogenesis, and maturation of the female germ cell are critical issues in reproductive research that define the central theme of this program. These issues are also of fundamental importance to improving the diagnosis, understanding and treatment of diseases and dysfunctions impacting fertility, and thus are of direct relevance to human health. It is our expectation that the basic research projects presented in this P01 program proposal will have considerable impact, ultimately leading to improvements in human reproductive health.